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Inspection and Rework

1. Tools for inspecting assemblies:

Magnifying glasses 5 to 10X with halo lamps
Printed workmanship standards to eliminate confusion
10 to 30X microscope may be required for inspecting difficult parts such as fine pitch components and solder paste quality.
For high volume manufacturing automated visual inspection such as 3 dimensional laser scan or X-ray scan may be justified.

2. Techniques for Inspecting Assemblies:

Boards need to be tilted to examine under components when inspecting for trapped contaminants or J type leads. Also, if rework and touch-up are to be done along with inspection, a board positioning table may be useful.
Dental picks or pointed wood sticks are useful to verify the soldering joints of leaded components when in doubt. Use pick or stick to gently push the top edge of lead to check for joint attachment. This prevents damage to the board and lead. An unsoldered lead or one with cold joint will move when pushed and this needs to be touched-up.
Write down or log all defect types using a common vocabulary. This information should be useful to find common defects and trends so their source can be found and eliminated.

Good solder joint:

o Smooth and shiny. No voids, or blowholes.
o Good wetting of soldered joints
o A concave fillet shape and free from excessive solder deposition.

3. Touchup, Rework and repair:

o Rework or repair is required to be done on components/ boards that are not meeting the workmanship or performance standards.
o Rework tools that are commonly used are soldering irons, desoldering stations using suction, hot air jets, solder wick etc.
o Rework on SOs and other high lead count packages needs prior training and some practice on dummies will be helpful.
o Since leads and terminations of SMT packages are small, the thermal requirement is less compared to through-hole component pads. The components are to be removed only after ensuring that all leads are desoldered or solder has reflowed. If not done properly, the chances of damage to the pad area is significant.
o Before removing the component gently push it to check for complete solder melt.
o When the connecting solder melts on each lead or termination of the component, it's readily removed and replaced with a new one.
o SMT rework requires operator training as it requires new techniques and new tools.

4. Tools Required:

o Alcohol in dispensing bottle
o Cotton swabs
o R' , RMA' flux in a small dispensing bottle
o Dental picks or pointed wooden sticks
o Long, thin tweezers
o Solder wicks in sizes as required
o Fine tip temperature controlled soldering iron and spares as required
o Desoldering station with appropriate bits
o Manual hot air jet and nozzles and hot air rework station
o Solder paste and dispenser
o 24 gauge ( 0.015) flux cored solder wire
o Static free work station with wrist strap and proper grounding
o Workmanship guidelines





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